X-ray reflectivity measurements were performed on two different polyimide thin films synthesized from 2,2-bis(3,4-carboxyphenyl)hexafluoropropane dianhydride (6FDA) in 1,3-butadiene and n-butane. In 1,3-butadiene at 2.3 atm, the film thickness increased by 24-30%. However, the film thickness increased by only 10% in n-butane at 2.3 atm. Excessive increases-in film thickness were shown in 1,3-butadiene, but the decreases in film density were minor. The probability of the condensation of 1,3-butadiene in the films is indicated.